To date, mainly (intrusive) installation solutions have been known which have various disadvantages: the temperature sensor already has to be installed during construction of the installation as far as possible. If retrofitting is required, it is necessary either                for there to already be an installation opening for the sensor or        for a hole to be drilled and a thermowell to be welded in. The temperature sensor is inserted into this thermowell. This is not possible during operation, and accordingly the installation has to be temporarily switched off. A solution of this kind is known, for example, from the publication http://www.burnsengineering.com/local/local/uploads/files/snx_family.pdf20.        
In this document, the solution is referred to as being nonintrusive since the actual sensor is not immersed in the fluid.
In the case of nonintrusive sensor solutions, the sensor is placed on the outside of the tube and the fluid temperature is indirectly inferred by means of measuring the tube temperature. The problem faced here is that, on account of the ambient temperature which usually differs from the fluid temperature, a flow of heat is generated between the fluid and the surrounding area across the tube wall and any boundary layers, this flow of heat, by means of the upstream thermal resistances, causing corresponding temperature differences and therefore faulty temperature measurement.
In order that these differences remain as low as possible (as far as possible <1K), a thermal insulation is provided, inter alia, around the tube, said thermal insulation reducing the radial heat flow and therefore the error in temperature measurement. The problem faced here is that, in the case of known solutions, the thermal insulation in the vicinity of the sensor is interrupted owing to the required electrical and mechanical sensor connections and a portion of the heat flows away in the radial direction via these connections. This heat flow generates further measurement errors and secondly requires very good thermal coupling of the sensor at the curved tube surface. A procedure of this kind is, however, complicated to implement in the case of retrofit applications. This is particularly true since the electronics system which is connected by means of the sensor element (generally a Pt100) has to be electrically insulated from the tube wall. The embedding of said sensor element has to be designed primarily for electrical strength and can therefore only secondarily be optimized for good thermal contact. This necessarily leads to poor thermal coupling since electrically insulating materials also conduct heat poorly. In general, the Pt100 is surrounded by a few millimeters of ceramic powder in the cap of the temperature sensor.
Alternative solutions in which the sensor lines are initially routed some way along the tube in the axial direction in order to reduce the corresponding heat flow in the vicinity of the sensor also have disadvantages. Firstly, said solutions are associated with considerable installation costs. In addition, the problem of high thermal resistance between the Pt100 and the tube is not solved, but rather increases only the thermal resistance to the surrounding area owing to the closed insulation. Although the absolute accuracy in the stationary state is improved in this way, the response time of the sensor is not improved. However, rapid response of the sensor is likewise required.
Wireless methods for temperature measurement are known from various applications, but not against the background of improved measurement accuracy in cases in which there are unavoidable and significant thermal resistances between the object of the temperature measurement and the actual measurement point.
The publication http://www.sengenuity.com/tech_ref/Smart_Grid_Solutions_Leveraging_SAW.pdf discloses wireless measurement for potential-free monitoring of the temperature of solid conductor and contact elements in switchgear installations by means of SAW sensors, without batteries or energy harvesting methods being required.
U.S. Pat. No. 8,152,367 B2 describes an intrusive temperature measurement operation in a closed container, without said container requiring a bushing.
The INTERNATIONAL JOURNAL ON SMART SENSING AND INTELLIGENT SYSTEMS, VOL. 4, NO. 2, JUNE 201, published under http://www.s2is.org/Issues/v4/n2/papers/paper10.pdf, discloses temperature measurement on rotating machines without mechanical contact.